An organic electroluminescence (EL) device is a spontaneous light emitting device which utilizes a principle that a fluorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied. Since an organic EL device of a laminate type driven under low electric voltage was reported by C. W. Tang et al. of Eastman Kodak Co. (for example, C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, P. 913, 1987), many studies have been conducted on organic EL devices using organic materials as constituting components. Tang et al. used tris(8-hydroxy quinolinol aluminum) for a light emitting layer and a triphenyldiamine derivative for a hole transporting layer. Advantages of the laminate structure include that the efficiency of hole injection into the light emitting layer can be increased, the efficiency of forming exciton which are formed by blocking and recombining electrons injected from the cathode can be increased, and exciton formed within the light emitting layer can be enclosed therein. For the structure of the organic EL device as in the example, a two-layered structure having a hole transporting (injecting) layer and an electron transporting and light emitting layer, a three-layered structure having a hole transporting (injecting) layer, a light emitting layer, and an electron transporting (injecting) layer, and the like are well known. To increase the efficiency of recombination of injected holes and electrons in the devices having such the laminate type structures, the structure of the device and a process for forming the device have been modified.
Known examples of the light emitting material include: a chelate complex such as a tris(8-quinolinolato)aluminum complex; a coumarin derivative; a tetraphenylbutadiene derivative; a distyrylarylene derivative; and an oxadiazole derivative. It has been reported that light in a visible region ranging from a blue color to a red color can be emitted from each of those light emitting materials, so the realization of a color display device is expected from each of them (see, for example, Patent Document 1).
In addition, in recent years, there has also been proposed that a phosphorescent material as well as a fluorescent material is used in the light emitting layer of the organic EL device (see, for example, Non-patent Document 1 and Non-patent Document 2). In this way, high luminous efficiency is achieved by utilizing a singlet state and a triplet state in excited states of the phosphorescent material in the light emitting layer of the organic EL device. When an electron and a hole recombine in the organic EL device, singlet excitons and triplet excitons are considered to be produced at a ratio of 1:3 owing to a difference in spin multiplicity. Accordingly, the use of a phosphorescent light emitting material is considered to achieve luminous efficiency three to four times that of a device using only the fluorescent material.
A constitution in which layers are sequentially laminated, for example, an anode, a hole transporting layer, an organic light emitting layer, an electron transporting layer (hole inhibiting layer), an electron transporting layer, and a cathode in the stated order has been used in such the organic EL device in order that a triplet excited state or a triplet exciton does not quench. A host compound and a phosphorescent compound have been used in the organic light emitting layer (see, for example, Patent Document 2 and Patent Document 3). Those patent documents relate to techniques each concerning a phosphorescent material emitting light having a color ranging from a red color to a green color. A technique concerning a light emitting material having a blue-based luminescent color has also been published (see, for example, Patent Document 4, Patent Document 5, and Patent Document 6). However, a device using any one of those materials has an extremely short lifetime. In particular, Patent Documents 7 and 8 each describe a ligand skeleton in which an Ir metal and a phosphorus atom are bonded to each other. The ligand skeleton described in each of those documents turns a luminescent color into a blue color, but a bonding force between the metal and the atom is so weak that the ligand skeleton is remarkably poor in heat resistance. Patent Document 7 similarly describes a complex in which an oxygen atom and a nitrogen atom are bonded to a central metal, but has no description concerning a specific effect of a group to be bonded to the oxygen atom, so the effect is unclear. Patent Document 8 discloses a complex in which nitrogen atoms in different ring structures are bonded one by one to a central metal. A device using the complex emits blue light, but has an external quantum efficiency as low as around 5%.
Meanwhile, research has been conducted on a transition metal complex compound having a metal carbene bond (which may hereinafter be referred to as “carbene complex”) in recent years (see, for example, Patent Documents 9 to 18 and Non-patent Documents 3 to 11).
The term “carbene” refers to dicoordination carbon having two electrons in an sp2 hybrid orbital or a 2p orbital. The carbene can take four kinds of structures depending on a combination of an orbital which the two electrons enter and the orientation of a spin. The carbene is typically singlet carbene composed of an sp2 hybrid, occupied orbital, and an empty 2p orbital.
A carbene complex, which has a short lifetime and is unstable, has been conventionally used as a synthesis conversion agent to be added to a reaction intermediate or olefin of an organic synthesis reaction. In about 1991, a stable carbene complex composed of a heteroaromatic ring structure and a stable carbene complex composed of a non-aromatic ring structure were found. Further, after that, it has been found that a non-cyclic carbene complex can be stably obtained by stabilization with nitrogen and phosphorus. In addition, the performance of a catalyst can be improved by bonding the non-cyclic carbene complex as a ligand to a transition metal. Accordingly, in a catalytic reaction in organic synthesis, expectations on a stable carbene complex have been raised in recent years.
In particular, in an olefin metathesis reaction, the addition or coordination of a stable carbene complex has been found to improve the performance of a catalyst significantly. In addition, researches on, for example, an improvement in efficiency of a Suzuki coupling reaction, the oxidation or selective hydroformylation reaction of an alkane, and an optically active carbene complex have been developed in recent years. Accordingly, the application of a carbene complex to the field of organic synthesis has been attracting attention.
In addition, specific examples of a complex having a carbene iridium bond are described in Non-patent Document 12 (tris(carbene) iridium complex composed of a non-heterocyclic carbene ligand) and Non-patent Document 13 (monodentate monocarbene iridium complex) to be described below. However, none of the documents describes the application of those complexes to, for example, the field of an organic EL device.
In addition, Patent Document 9 discloses the synthesis of an iridium complex having a carbene bond, the luminous wavelength of the complex, and the performance of a device using the complex. However, the complex has low energy efficiency and low external quantum efficiency, and its luminous wavelength is distributed to an ultraviolet region, so the complex has poor luminous efficiency. Therefore, the complex is not suitable for a light emitting device emitting light having a wavelength in a visible region such as an organic EL. In addition, the complex cannot be used in vacuum deposition because of, for example, its low decomposition temperature and its large molecular weight, and the complex decomposes upon vapor deposition, so the complex involves a problem in that an impurity is mixed upon production of a device.
Further, Patent Documents 10 to 18 describe various complexes each having a carbene bond, and each disclose a blue light emitting complex. However, the energy efficiency and external quantum efficiency of the blue light emitting complex are low, and none of the documents mentions an increase in emission lifetime.
Meanwhile, Patent Documents 19 and 20 each disclose, as a method of increasing the lifetime of a tris(2-phenylpyridine-N,C2) iridium complex, the crosslinking of three 2-phenylpyridine-N,C2 group sites in a tripod manner. However, the documents each report only a tripod crosslinked site having a benzene ring skeleton, so none of the documents has achieved a significant increase in lifetime of the complex. In addition, none of the documents describes a guideline for the emission of blue light.
Patent Document 1: JP-A-08-239655
Patent Document 2: U.S. Pat. No. 6,097,147
Patent Document 3: WO 01/41512
Patent Document 4: US 2001/0025108
Patent Document 5: US 2002/0182441
Patent Document 6: JP-A-2002-170684
Patent Document 7: JP-A-2003-123982
Patent Document 8: JP-A-2003-133074
Patent Document 9: WO 05/019373
Patent Document 10: US 2005/0258433
Patent Document 11: US 2005/0258742
Patent Document 12: US 2005/0260441
Patent Document 13: US 2005/0260444
Patent Document 14: US 2005/0260445
Patent Document 15: US 2005/0260446
Patent Document 16: US 2005/0260447
Patent Document 17: US 2005/0260448
Patent Document 18: US 2005/0260449
Patent Document 19: US 2005/0170206
Patent Document 20: US 2005/0170207
Non-patent Document 1: D. F. OBrien and M. A. Baldo et al “Improved energy transfer in electrophosphorescent devices” Vol. 74, No. 3, pp 442-444, Jan. 18, 1999
Non-patent Document 2: M. A. Baldo et al “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” Applied Physics letters Vol. 75, No. 1, pp 4-6, Jul. 5, 1999
Non-patent Document 3: Chem. Rev. 2000, 100, p39
Non-patent Document 4: J. Am. Chem. Soc., 1991, 113, p361
Non-patent Document 5: Angnew. Chem. Int. Ed., 2002, 41, p1290
Non-patent Document 6: J. Am. Chem. Soc., 1999, 121, p2674
Non-patent Document 7: Organometallics, 1999, 18, p2370
Non-patent Document 8: Angnew. Chem. Int. Ed., 2002, 41, p1363
Non-patent Document 9: Angnew. Chem. Int. Ed., 2002, 41, p1745
Non-patent Document 10: Organometallics, 2000, 19, p3459
Non-patent Document 11: Tetrahedron Aymmetry, 2003, 14, p951
Non-patent Document 12: J. Organomet. Chem., 1982, 239, C26-C30
Non-patent Document 13: Chem. Commun., 2002, p2518